Extensive investigations of the structure of the chromatin core particle have been carried out by a variety of physicochemical techniques. We have determined the positions and relative susceptibilities of nuclease cleavage sites within the core particle for DNases I and II, an endonuclease from Aspergillus and micrococcal nucleases. Resistance to cleavage of the central portion of core particle DNA by those nucleases which require divalent cations likely arises from binding of the basic regions of the smaller histones to those DNA sites. The amino terminal ends of the histones appear to also bind DNA at the entry and exit points for the nucleic acid on the nucleosomes. Crosslinking experiments with core particles labeled at the 5' DNA termini suggest that H3 is located at the ends of the DNA. The role of various histones in organization of the structure of the core particle has been investigated. H3 and H4 (but not H2A and H2B) fold 140 base pair DNA into a compact structure (S equals 9.2, 75 A diameter). These arginine rich histones can also fold SV40 DNA and induce torsional constraints on the DNA equivalent to those induced by all four smaller histones. A crosslinked octamer of the four smaller histones has been prepared and is used as a model for the protein core of the nucleosome. BIBLIOGRAPHIC REFERENCES: Bina-Stein, M. and Simpson, R.T. (1977) Specific folding and contraction of DNA by histones H3 and H4. Cell, in press. Piatigorsky, J., Horwitz, J. and Simpson, R.T. (1977) Partial dissociation and renaturation of embryonic chick delta-crystallin. Characterization by ultracentrifugation and circular dichroism. Biochem. Biophys. Acta 490: 279-289.